KINETICS AND MECHANISM OF HYDROLYSIS OF
TRI-4-CHLOROTHIOPHENYL PHOSPHATE ESTER VIA CONJUGATE
ACID SPECIES
Asha Verma1 and Firdous Andleep2*
1
Professor of Chemistry, 2Research Scholar
Department of Chemistry, Govt. Science and Commerce College, Benazir, Bhopal, India.
ABSTRACT
Hydrolysis of Tri-4-chlorothiophenyl phosphate ester has been carried
out in the region of 0.1-7.0 mol dm-3 HCl at 980C. Acid log rate profile
has a maximum at 4.0 mol dm-3 HCl and then decreases after 4.0 mol
dm-3 HCl. This decrease in rate is attributed to water activity and
negative effect of ionic strength. The theoretical and observed rates
estimated from second empirical term of Debye- Huckle equation have
been found to be in close agreement. Bimolecularity of the reaction has
been determined from Arrhenius parameters and molecularity data.
The triester involves P – S bond fission which is strengthened by
comparative kinetic data.
KEYWORDS: Tri-4-chlorothiophenyl phosphate, bimolecularity, trimester, Arrhenius
parameter.
INTRODUCTION
Organophosphates having C-S-P linkage are important class of compounds that find their
applications in many fields. Besides their antiviral activity[1] and radioactive tracer
techniques[2], they are used in biological investigations, insecticidal activity[3] and textile
commodities.[4] Taking this in view Tri-4-chlorothiophenyl phosphate was chosen for kinetic
study as this compound is reactive via its different reactive species, depending upon the
experimental conditions.
Volume 6, Issue 3, 1247-1252. Research Article ISSN 2277– 7105
*Corresponding Author
Firdous Andleep
Research Scholar,
Department of Chemistry,
Govt. Science and
Commerce College, Benazir,
Bhopal, India.
Article Received on 13 Jan. 2017,
Revised on 02 Feb. 2017, Accepted on 23 Feb. 2017
EXPERIMENTAL
Tri-4-chlorothiophenyl phosphate was prepared by using PCl5 as phosphorylating agent[5,6,7]
The ratio of 3:1 thiol and PCl5 was employed for this preparation.
Theoretical: C = 45.244, H = 2.5314, Cl = 22.2589, P = 6.4835, S = 20.132, O = 3.3485
Observed: C = 45.11, H = 2.325, Cl = 21.908, P = 6. 246, S = 20.03, O = 3.186.
The kinetic analysis of the present triester was carried out by hydrolyzing it in the range of
0.1-7.0 mol dm-3 HCl and 1.24 – 7.46 pH in aqueous 10% dioxan-water medium (V/V) at
980C. The constant ionic strength was maintained using appropriate mixtures of HCl and
NaCl.
RESULTS AND DISCUSSION
From the hydrolysis of present triester, pseudo first order rate coefficients are found to
increase with the increase in acid molarity upto 4.0 mol dm-3 HCl in the range of 0.1-7.0 mol
dm-3 HCl. Further increase in acid molarity after 4.0 mol decreases the rate constant due to
negative effect of ionic strength. Hydrolysis at three different ionic strengths i.e.,1.0, 2.0 and
3.0µ is denoted by linear curve that makes a negative slope with acid axis indicating the
presence of acid catalysis (slope values KH+ = 47.77, 31.57 and 25.91 for1.0, 2.0 and 3.0µ
respectively. Each of these lines may be represented by
Ke = KH+. CH+ ……….. (I)
Where
Ke, KH+ and CH+ are experimental acid catalysed and specific acid catalysed at that ionic
strength and and hydrogen ion concentration respectively.
Figure 1: Acid catalyzed hydrolysis of Tri-4-chlorothiophenyl Phosphate at constant
[image:2.595.134.464.573.721.2]From the study of ionic strength effect, the total rate contributed by conjugate acid species
and neutral species can be calculated by second empirical term of Debye- Huckle equation.[7]
Ke = KH+. CH+ + KN ………….. (II)
Table 1: Estimated and experimental rates of hydrolysis of Tri-4-chlorothiophenyl
phosphate at 98⁰c
HCl mol dm-3
KN ×
103 min-1
KH+CH+
× 103 min-1
Ke×103
min-1 (aH2O)
n Ke×10 3
min-1 Estm.
Ke×103
min-1 Expt.
3+log Ke
Estm.
3+log Ke
Expt.
0.1 18.0 6.071 24.071 24.071 24.01 1.381 1.380
0.5 18.0 26.8 44.85 44.85 44.63 1.652 1.649
1.0 18.0 46.13 64.13 64.13 64.25 1.807 1.808
1.5 18.0 59.36 77.36 77.36 76.98 1.888 1.886
2.0 18.0 67.92 85.92 85.92 85.56 1.934 1.932
2.5 18.0 72.69 90.69 90.69 90.79 1.957 1.958
3.0 18.0 74.99 92.99 92.99 92.83 1.968 1.967
3.5 18.0 75.08 93.08 93.08 93.00 1.969 1.968
4.0 18.0 77.62 95.62 95.62 95.89 1.981 1.982
5.0 18.0 67.76 85.77 (0.155)1 65.32 68.91 1.815 1.838 6.0 18.0 59.84 77.84 (0.211)2 40.65 42.34 1.609 1.627 7.0 18.0 51.40 69.40 (0.279)3 25.482 26.59 1.406 1.425
From the above table it is observed that estimated and experimental rates agree well with
each other and the decrease in rate after 4.0 molarity of HCl may also be attributed to water
activity along with negative salt effect.
The rate law may be formulated as
(1) In the region from 0.1 to 4.0 mol dm-3HCl
Ke = 62.66 × 10 -3 min-1 CH+. exp. (-0.133 × 2.303). µ + 18.00 × 10 -3min-1
(2) In the region > 4.0 mol dm-3 HCl
Ke = 62.66 × 10 -3 min-1 CH+. exp. (-0.133 × 2.303). µ. (aH2O)n + 18.00 × 10 -3 min
Where (aH2O)n is water activity and n is an integer.
The magnitude of Arrhenius[8] parameters determined for the hydrolysis at 3.0 and 5.0 mol
Table 2: Arrhenius parameters for the hydrolysis of Tri-4-chlorothiophenyl phosphate
in acid media
HCl mol dm-3
Parameter Energy of
Activation "E" K cal mol-1
Frequency factor "A"
Sec-1
Entropy of Activation - ∆S≠ e.u.
3.0 4.58 2.06×10-3 51.956
5.0 1.35 2.03×10-3 51.551
Bimolecular nature is further supported by Zucker- Hammet9 (0.330), Hammet10 (0.168) and Bunnet (ω = 10.65, ω*
= 4.51) plots. Bunnet – Olsen parameter (ϕ = 1.37) which is greater
than 0.58 suggests that water is involved as proton transfer agent in the rate determining step.
The effect of solvent shows a significant rise in rates due to better proton donating properties
of dioxin.[13] Hence the solvent effect (table not shown) may be taken in accordance with Chanley’s.[13]
observation, indicating the formation of a transition state in which charge is
dispersed. Bimolecular nature is further supported by comparative kinetic data involving P-S
bond fission.
Mechanism
Hydrolysis of Tri-4-chrorothiophenyl phosphate via conjugate acid species, may be
formulated as follows.
(1)Formation of conjugate acid species of triester by fast pre-equilibrium proton
transfer
O
S P S
Fast Cl
S
Cl
Cl Cl + H3+ O H2O
Cl
O
S P S
S
Cl H
Cl +
(2)Bimolecular heterolysis of P-S bond of the acid species by SN2 (P)
O
S P S
SLOW Cl
O H
H
H
Cl
O
Cl
S
Cl H
Cl
Transition State S...P S
H
H
δ δ
O
Cl S
Cl SH + O P S
O
S H
H
Cl
Cl
Fast proton transfer
O
S
HO Cl
Cl
P S + H
Fast
Diester
This is followed by the hydrolysis of diester into monoester and then into inorganic
phosphate.
CONCLUSION
New research in the field of kinetics of phosphate esters can help an academician to design an
orthophosphate pesticide with low toxicity and discovery of novel bioactive molecules. So
that they should be more specific in their action and instrumental in raising crop yields with
high rates of degradation by soil bacteria of several genera and decompose eventually. Also
ACKNOWLEDGEMENT
The authors are thankful to the Dept. of Chemistry, Govt. Science and Commerce College,
Benazir, Bhopal for providing the required lab facilities for carrying out the research work.
REFERENCES
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5. John, A.D., (1923), St., U.S. Pat. Z., 462: 306.
6. Shuman, R.L., (1938), U.S. Pat. 2, 133: 310
7. Williamson, Ann., 1854; 92: 316.
8. Arrhenius, S. Z., (1889), Phys. Chem., 4: 226.
9. Zucker, L. and Hammet, P. , (1932), J. Am. Chem. Soc., 61: 2791.
10.Hammet, L.P., (1940), Physical Organic Chemistry, McGraw-Hill, London, 335.
11.Bunnet, J.F., (1961), J. AM. Chem. Soc., 83: 4956.
12.Bunnet J.F. and Olsen, F.P., (1966), Can. J. Chem., 44: 1917.